In recent years, reduction of amount of waste processing solutions is strongly desired in the medical fields from the standpoints of environmental protection and space savings. Techniques relating to photosensitive heat-developable photographic materials for use in medical diagnosis and photomechanical processes are required which enables efficient exposure by a laser image setter or laser imager and formation of a clear black image having high resolution and sharpness. The heat-developable photosensitive photographic materials can provide users with a simple and non-polluting heat development processing system that eliminates the use of solution-type processing chemicals.
Although the same is required also in the field of general image-forming materials, the image for medical diagnosis in particular must be finely drawn and therefore, high image quality with excellent sharpness and graininess is needed. Moreover, in view of diagnostic convenience, an image of cold black tone is preferred. At present, various hard copy systems using a pigment or a dye are commercially available as a general image-forming system, such as ink jet printer and electrophotography, however, these are not a satisfactory output system for the medical-use image.
On the other hand, thermal image forming systems using an organic silver salt are described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, B. Shely, Thermally Processed Silver Systems, and Sturge, V. Walworth and A. Shepp (compilers), Imaging Processes and Materials, 8th ed., page 2, Neblette (1989).
In particular, heat-developable photosensitive materials generally have a photosensitive layer comprising a binder matrix having dispersed therein a catalytic amount of a photocatalyst (for example, silver halide), a reducing agent, a reducible silver salt (for example, organic silver salt) and if desired, a color toner for controlling the silver tone. The heat-developable photosensitive material after image exposure is heated at a high temperature (for example, 80° C. or more) to bring about an oxidation-reduction reaction between the reducible silver salt (acting as an oxidizing agent) and the reducing agent and thereby form a black silver image. The oxidation-reduction reaction is accelerated by the catalytic action of a silver halide latent image produced by the exposure. Therefore, the black silver image is formed in the exposed area. This is disclosed in many publications including U.S. Pat. No. 2,910,377 and Japanese Patent Publication No. 4924/1968. As a medical image forming system by using a heat-developable photosensitive material, “FM-DP L” (Fuji Medical Dry Imager) is put on the market.
For the production of a thermal image forming system using an organic silver salt, there are two methods, that is, solvent application; and application of a coating solution which contains, as a main binder, an aqueous dispersion of fine polymer particles and then drying. The latter method needs only a simple production equipment and is suited for mass production, because a step for collecting a solvent is unnecessary.
Such an image forming system using an organic silver salt is however accompanied with such a serious problem as a deterioration in image shelf life after development, particularly, deterioration in printout when it is exposed to light because of a lack of a fixing step. As a means for improving such a deterioration in printout, a method of making use of AgI formed by the conversion of an organic silver salt is disclosed in U.S. Pat. No. 6,143,488 or European Patent No. 0922995. The above-disclosed method of using iodine for conversion of an organic silver salt however cannot be adopted as a practical image forming system, because sensitivity attained by the method is insufficient.
In addition, photosensitive materials making use of AgI are described in WO97-48014, WO97-48015, U.S. Pat. No. 6,165,705, Japanese Patent Laid-Open No. 297345/1996, and Japanese Patent No. 2785129. They have not attained satisfactory levels of sensitivity and fogging and are insufficient for practical use as a photosensitive material to be exposed to laser light. There is therefore a demand for the development of a method fully utilizing silver halide having a high silver iodide content.
Although an image forming method and a photosensitive material using a blue to ultraviolet laser light are disclosed in Japanese Patent Laid-Open No. 1 305213/2000, they are low in a silver iodide content and insufficient in sensitivity.